1. Field of the Invention
This invention relates to a semiconductor process, and particularly to an immersion lithography process and a patterning process.
2. Brief Description of Related Art
Photolithography is one of the most important technologies in semiconductor production. All patterns or doping areas regarding to metal oxide semiconductor device are defined by photolithography. In general, photolithography process includes photoresist coating, light exposure and development. In the exposure step, exposure light beams travel through a mask to reach a photoresist layer or directly reach the photoresist layer to generate photochemical reaction at exposed sections of the photoresist layer. After baking and developing steps, mask patterns are transferred to the photoresist layer to form a patterned photoresist layer.
As the integration of integral circuits increases, the device size of the integral circuits decreases. In order to meet the requirement in reduction of device size, an immersion lithography process has been developed. Currently, this immersion lithography process is conducted in liquid phase. By taking the advantage that the refractive index of liquid is higher than that of air, the wavelength will be shortened while light travels through liquid. The resolution of exposure is thereby increased to achieve the reduction in device size.
However, the immersion lithography process has several critical concerns to be overcome, such as interaction between the immersion liquid and the photoresist layer and the control of micro-bubbles in the liquid. Specifically when the exposure step is conducted, part of chemicals enters the liquid from the photoresist layer due to the contact with the liquid, which deteriorates the photoresist layer. Particularly, when photo-acid generator (PAG) contained in the photoresist layer diffuses into the immersion liquid during the exposure step, the PAG concentration will be lowered. As a result, the developed photoresist patterns have T-top problem. As such, the process resolution, the process window, and the critical size and uniformity of the photoresist patterns are adversely affected.
One approach has been proposed to solve the T-top problem in the art by forming a barrier layer over the photoresist layer. The barrier layer does not contain photo-acid generator, and is used to stop the photo-acid generator from diffusing into the immersion liquid. The formation of the barrier layer is complicated as the property of the barrier layer must be unsolvable in the immersion liquid during the exposure step while solvable in tetra-methyl-ammonium hydroxide (TMAH) developing agent. Furthermore, forming the barrier layer on the photoresist layer adds cost to the total production.